The hint was found in a screenshot of an email that was published on the official Twitter of Game Pass. The message tells about a certain game that will be available to subscribers of the service in the near future, but its name is hidden.
However, if you change the contrast and gamma of the attached image, you can see a caption under the main text that merges with the background: “THE SLAYER IS COMING” (“the Executioner is coming”).
Neither DOOM 2016 nor its sequel are currently available in Game Pass on PC and consoles. The first part was already in the service’s library, but it was removed in May 2020.
Apparently, after the announcement that Microsoft bought Bethesda, more and more games from the publisher will appear in the Xbox subscription service.